1. Field of the Invention
This invention relates to extruded tubing and, in particular, to extruded tubing having either a constant or variable inner diameter and a variable outer diameter.
2. Prior Art
It has long been known in the art to form tubing having a corrugated wall structure to permit the tube to be easily flexed without buckling. However, such tubing structures have both interior and exterior surface irregularities and, as a result, when certain fluids are flowed therethrough, unwanted resistance to flow is affected by means of the irregular internal configuration. In addition such surfaces are difficult to clean.
To overcome these problems, Weigl, in U.S. Pat. No. 3,858,615, developed a kink-resistant hose construction. Weigl's hose is a flexible tube having a very thin wall relative to the inside diameter of the tube, and a series of axially spaced reinforcing rings of the same material as the tube integrally formed on the outer surface of the tube wall. The inner surface of the tube wall, being smooth and substantially cylindrical enables unimpeded flow therethrough and facilitates easy cleaning. The unsupported portion of the tube wall between successive reinforcing rings flexes readily. When a severe bending force is applied, the wall portion between rings folds inwardly toward the axis of the tube. The reinforcing rings are spaced such that such extreme bending brings adjacent reinforcing rings in contact with each other to prevent closure of the passage.
While Weigls kink-free tubing established the value of a tubing having a uniform interior diameter along its length and a periodically varying outer diameter, until now the method for making such tubing has been limited to molding. Molds are useful for making short sections of tubing but impractical for making long lengths of tubing. It is, therefore, desirable to provide a method for continuously extruding a tube having a uniform inner diameter and a variable-outer diameter.